[deathanatos]

… that incorrectly detects devtools as open, when it isn't (false positive), and fails to detect devtools when it is open in a separate window (false negative)…

Yeah, as other posters hint at, vertical tabs. But it isn't just vertical tabs that'll trigger it, any sidebar will, including native/vanilla ones; AFAICT it's just looking at the client area being less than the window by some threshold.

[wolpoli]

It detects Microsoft Edge's Sidebar too, which opens on hover. It's going to create a lot of false positives.